Myocardial ischemia, a leading cause of mortality, involves oxygen starvation of the myocardium. Myocardial ischemia can lead to myocardial infarction if left untreated. Early detection of myocardial ischemia provides the opportunity for a wide range of effective therapies such as revascularization, neural stimulation, and drug delivery to reduce cardiac workload or improve cardiac circulation. Unfortunately, many episodes of myocardial ischemia do not cause excessive pain or other noticeable warning signs, and often go undetected.
An electrocardiogram (ECG) or electrogram (EGM) presents a PQRST waveform sequence that characterizes the cyclical cardiac activity of a patient. The ST segment, also associated with the repolarization of the ventricles, is typically close in amplitude to the baseline, i.e., isoelectric amplitude of the signal sensed in the TP or PQ segments. During episodes of myocardial ischemia, the ST segment amplitude often deviates from the baseline. Accordingly, deviation in the ST segment is often used to identify an occurrence of myocardial ischemia. Hence, an implantable medical device may be equipped with an ischemia detector that indicates an ischemic condition based on elevation or depression of the ST-segment relative to a baseline. Alternatively, the ischemia detector may rely on a measure of heart activity or patient workload.
Unfortunately, the use of the ST segment as an indicator of ischemia can be unreliable. The ST segment may deviate from the baseline due to other factors, causing false indications of myocardial ischemia. For example, the ST segment may deviate from the baseline due to changes in the overall PQRST complex, possibly caused by axis shifts, electrical noise, cardiac pacing stimuli, drugs and conduction aberrancy that distorts the PQRST complex. In addition, the occurrence of cardiac ischemia may not manifest as changes in the ECG or EGM signals. Consequently, the reliability of the ST segment as an indicator of myocardial ischemia can be unacceptably low.
U.S. Pat. No. 6,128,526 to Stadler et al. describes an ischemia detector that observes variation in the ST segment to identify an ischemic condition. To improve reliability, the detector is designed to filter out ST segment variations caused by factors other than ischemia, such as axis shift, electrical noise, cardiac pacing, conduction aberrancy, and other distortions in the overall PQRST complex. Nevertheless, the sensitivity and specificity of ischemia detection based on observation of ST-segment changes is suboptimal. Accordingly, there continues to be a need for a simplified system capable of automatically and reliably detecting myocardial ischemia.